Methane VS Pentane as a Calibration Gas

Hey everyone,

I made up a video on some of the dangers of using Pentane as a calibration gas and why it is important to use Methane or a Pentane simulant (Methane formulated to act like Pentane). Hope it's informative and helpful for you guys to actually see what the problem is with using real Pentane. When it is poisoned, the sensor may still calibrate just fine to Pentane, but will have a hugely reduced response to Methane. So if you go out to a natural gas leak, it may be showing 20% or so of what is really there.

Thanks for posting that. In the video you talk about sensor poisoning, but isn't it also true that as LEL sensors age they just become differentially less sensitive to methane compared to other gasses (e.g. pentane)?

Results

Hey StephanK,

It's something I've heard before, and the theory behind it is solid, but it's not something I've tested myself, nor have I seen numbers from a test on it. But having said that, it's something I'd DEFINITELY like to test, and if that's proven, it would toss out all of the correlation factors we currently think we know, that going for ANY monitor manufacturer. I'll start working on something to test it. Maybe take an LEL sensor, run it through constant exposure to methane to burn it down, and check Pentane span numbers vs Methane span numbers on it at different intervals to see if the correlation factors hold as the sensor dies out? Let me know if you've got any suggestions, LEL sensors are expensive, so I'd like to get the test right the first time.

Personally I think you're right, to me it makes complete sense, but I don't want to say yes 100% until I know. In my mind, it may depend on where the LEL sensor is used and around what. Old sensors have most likely been around a lot of different inhibitors and poisons over time, and they're likely to have accumulated small amounts of the poison on the internal bead. In that case, it's actually a case of accumulated poisoning over time, and not the age of the sensor specifically. Likely the two would correlate almost every time, so I'm really curious to see how an LEL sensor would do in a completely clean environment, with ONLY Pentane and Methane being run on it. To my knowledge, no one has done a test that way yet, so it'd be an interesting one for us to do.

Again, please let me know if you've got any suggestions, I can always use the help.

I watched your video. Remember that most LEL sensors are dependent on a O2 level of 20.9 %. It looked like the O2 level was around 16%. Lots of times when the O2 is out of range it effects the LEL sensor and weird readings happen. I have seen this happen on Q Rae's, Industrial Scientific, BW's and MSA's. I do a demo for our Hazmat team showing how oxygen enriched and deficient atmospheres will screw up the LEL reading on just about any instrument that depends on burning the product for a reading on the LEL readout. We use instruments that are calibrated to Pentane and Methane. I have had no problem with the readings as long at the calibration gas has a 20.9 % mix of oxygen. If you use a balance with Nitrogen and no O2 it will give false readings. Hope this helps. Stay safe

Sorry it's taken so long to reply, but I wanted to do some research first.

I'm curious what testing you've done showing the readings of the LEL sensor to change based on the oxygen content. I'd like to duplicate them here if possible, and we blend gas here, so I can make any mixes necessary to replicate those tests.

From what I'd heard and seen before, O2 really isn't a problem until it gets down to the less than 10% range. I called up each of the manufacturers I do repairs for, and they all said it's not an issue till under 8-10% oxygen, but not a single one of them could tell me why. So what I did was track down Dr. Werner Haag, who used to work for RAE Systems. Dr. Haag is really a cool guy in that all he did at RAE was test things on sensor response and write up the results. He's one of the few guys out there that knows things because he's actually done the research, not because he read it in the book. In the case of the RAE PID Guide, as well as most of their Technical and Application Notes, he DID the writing, which I find pretty cool because a copy is on my shelf next to me.

"James,

The answers people gave you are more or less correct, but I can expand on it. I have personally done the tests of LEL sensors with reduced oxygen, so I can verify the results. I just can't show you the data because I don't have access to them any more. What I found was that the response is independent of oxygen concentration as long as you are above a minimum amount. If you take 50% LEL (2.5 vol%) methane, you need only about 5% oxygen to get full response, and if you use 1 vol% (20% LEL), you can go something lower than 4% oxygen. My interpretation is that the reaction is fast, and stoichiometric. The reaction is:

2O2 + CH4 ---> CO2 + 2H2O

So you need twice as much oxygen as methane. For 100% LEL, which is 5 vol% methane, you need 10% oxygen. That's where the 8-10% recommendation comes from. In reality alarms are set at 10-20% LEL, or 0.5-1 vol%, so you really only need 1-2% oxygen. Equipment manufacturers say 8-10% because it covers all explosive levels of methane, and that way they are safe.

Now, there are varying sensor designs that may have different results, but I believe these conclusions apply to the standard catalytic bead sensors used in most portable LEL sensors."

So I talked to him about the increase in flammability that comes with an enriched oxygen atmosphere, as well as the decrease in flammability from a decrease in oxygen, here was his response:

"When you hear that something is "more flammable" in enriched oxygen environments, it means that it can be ignited at lower temperature, so the risk is greater for accidental ignition. That's why intrinsic safety tests on instruments only apply to 20.9% O2; the certification is based on the maximum temperature or current that can be achieved in the worst case by the instrument, and they can't test all O2 levels so they take the most common. But the LEL signal depends not on how easily a material can be ignited, but rather on the total amount of heat that is released when it does burn. That depends primarily on the type of chemical and the amount present, not on how easily you get the fire started."

Sorry to throw all the quotes out there, but I figure it's better that everyone know these things rather than get it second hand from me. Basically, as long as the LEL sensor has enough oxygen to perform the operation of burning the chemical (2x as much O2 as CH4) you should get a full reaction. The rest of the oxygen isn't actually used in the sensor at all. The response you're seeing in the LEL sensor is the energy released by the Methane when it is actually burned off, and that shouldn't change regardless if there's 16% O2 or 20.9% O2, because it's reliant on the chemical itself.

So that's the chemistry talking, but all of that aside, would you mind elaborating on the tests you've done? If I can repeat them here, it'd be a good time to get them on video and show everyone. I've got all the gas necessary, I'd just have to know what instruments and what concentrations you were using.